Two-shaft type automatic transmissions for motor vehicles are known which combine a torque convertor and a shift change gear mechanism. The power transmission route of the shift change gear mechanism is switched by the selective operation of a plurality of frictional engaging elements such as clutches or brakes, etc., and constructed so that the gear stage is automatically switched in accordance with the driving condition of the vehicle. This type of automatic transmission has an input shaft for inputting engine output over the torque convertor, and an output shaft parallel to the input shaft. For example, Japanese Unexamined Published Patent Application (hereinafter referred to as Kokai) No. 62-93546 discloses an automatic transmission which has an input shaft connected to the turbine of the torque converter and an output shaft disposed parallel to the input shaft. A pair of planetary gear mechanisms, which serve as a shift change gear mechanism, is disposed between a partition wall portion and a rear wall portion of the shift gear casing over the input shaft.
In this type of transmission, in which the planetary gear mechanism is disposed over the input shaft, the mechanism is constructed with a helical gear. Because thrust load is generated following the operation of the planetary gear mechanism, thrust load is applied to the rear wall portion of the shift gear case in the direction of the shape of the helical gear. For this reason, it is necessary to increase the thickness of the rear wall portion, etc., and to improve the strength and rigidity of this portion. This tends to increase the weight and size of the shift gear mechanism.
Also, in order to obtain a plurality of gear stages by connecting the rotational elements of one of the pair of planetary gear mechanisms disposed over the input shaft, connecting members are necessary, which lead to an increase in the number of parts.
Therefore, it is an object of the present invention, in an automatic transmission having an input shaft, an output shaft parallel to the input shaft and a pair of planetary gear mechanisms disposed over the input shaft, to effectively transfer the thrust load generated by the operation of the pair of planetary gear mechanisms to the transmission casing. It is a further object of the present invention to keep the number of parts connecting with the rotational elements of each of the planetary gear mechanisms low.
In the above type of automatic transmission, hydraulic clutches are known, which serve as frictional engagement elements. Such as disclosed in Kokai 62-52249. In the disclosed clutch, in order to improve the releasability of the clutch, a centrifugal balance chamber is formed on one side of the clutch piston opposed to the hydraulic chamber.
In such known clutches, in the side portion of the clutch drum which forms the hydraulic chamber with the other side of the piston, elements for transmitting power to the clutch drum are provided, such as a one-way clutch for regulating the rotation of the clutch drum. A hydraulic clutch which serves as a power transmission device and contains a one-way clutch as above has increased axial dimensions, and thus causes the size of the entire transmission to increase.
It is, therefore yet another object of the present invention, to provide an automatic transmission in which, when a power transmission device such as a clutch and the power transmission elements for the clutch are aligned above the axis of the transmission, the power transmission elements can be arranged so as not to invite any increase in size of the transmission.